Gas separation is accomplished by processes such as pressure swing adsorption (PSA), vacuum swing pressure adsorption (VPSA), and temperature swing adsorption (TSA). These processes use vessels or beds containing one or more layers of adsorbent materials that adsorb one or more unwanted gases and produce the desired product gas. During the process cycle, the adsorbent materials adsorb the unwanted gases and then are regenerated by the removal of the unwanted gases by pressure, vacuum or temperature swings. Typical adsorbents used in these processes include zeolites, alumina and silica and combinations thereof.
Repeated use of an adsorbent causes degradation and eventually renders the adsorbent materials ineffective, such that the adsorbent materials need to be replaced periodically. Degradation of the adsorbent results in plant capacity decline and plant inefficiency, which is costly and time consuming. Often, adsorbent degradation is not detected until the product quality and/or overall plant performance is affected.
Currently, the testing of adsorbents is done by taking samples of the used adsorbent and subjecting them to various performance tests (including capacity measurements) and/or spectroscopic studies that are generally best performed in a laboratory or by a third party off-site. These methods of testing are slow and costly. It would be desirable to have a method of testing adsorbent degradation and quality that is quick and accurate and able to be conducted on-site at the separation plant.
The test methods currently in use include a Karl Fischer titration (KF) that determines trace amounts of water using potentiometric or coulometric titration. The application of KF methods to adsorbents such as zeolites requires system modifications whereby the sample is heated to high temperatures (about 1000° C.) and the water released is swept using a purge gas through a conventional KF cell. KF methods can specifically test for water content, unlike other loss on ignition (LOI) thermal methods that detect the loss of any volatile substance. However, the KF methods require the use of equipment that is very costly and not conducive to conducting the analysis outside of a laboratory, nor can they test for any other type of damage other than moisture damage.
LOI methods are based on the change in mass as a result of heating a sample under specified conditions. The LOI is expressed as a weight percentage of the dry mass. For example, a sample is heated in a furnace to a high temperature (e.g., 1000° C.) and the difference in mass before and after the ignition process is used to calculate the LOI. The LOI test measures the release of all volatiles which are adsorbed by the sample and cannot specifically test for moisture contamination or assess sample performance.
A near-infrared (NIR) moisture analyzer (e.g., Model #KJT-100, Kett US, Villa Park, Calif., USA) measures moisture levels based on the principle that water absorbs certain wavelengths of light. An optical filter is used to select a wavelength that either absorbs moisture (e.g., 1200, 1450 and 1950 nm) or does not absorb moisture (e.g., 1300 nm). This wavelength serves as a reference and incident radiation is reflected off a sample and measured by a lead sulfide (PbS) detector. The ratio of absorbed light to reference light is proportional to moisture content in the sample. Similar to KF titration, the NIR moisture analyzer only detects water and will not detect any other types of damage to the adsorbent and is very expensive.
An electronic moisture balance (e.g., Model #EB-340MOC, Shimadzu Corporation, Columbia, Md., USA) determines moisture content in solid substances using a thermogravimetric method. Far-infrared radiation is applied to the surface of a sample to heat it, then the sample is weighed upon drying and compared to the original weight. For many adsorbents such as zeolites, the output of the infrared heaters in commercial moisture balances is insufficient to remove all of the adsorbed water. Furthermore, the moisture balance can only detect moisture content and is very expensive.
Japanese Patent No. JP3110444 describes a method for measuring the adsorption performance of a solid adsorbent wherein a measuring gas containing the adsorption component is introduced into a vessel filled with the solid adsorbent. The measuring gas exits the vessel and goes through an analyzer that analyzes the amount of the adsorption component adsorbed by the solid material. This patent does not teach a method that can differentiate between moisture contamination and other damage to the adsorbent.
U.S. Pat. No. 4,237,726 (Peterson et al.) describes a process for predicting the useful life of a respirator cartridge wherein the process measures the weight increase of a sorptive agent when exposed to a gas mixture of dry air and a preselected organic vapor. In this process, the breakthrough time of the cartridge is determined by using the measured breakthrough time of the preselected vapor. As with the other processes described in the prior art, this process does not determine the damage to the sorptive agent and does not even measure moisture contamination of the sorptive agent.
The prior art and commercially available equipment all lack the capability of distinguishing between moisture contamination and any other type of damage to an adsorbent. Most of the prior art also involves the use of cumbersome equipment that cannot be used on-site at a plant and the methods are time consuming, expensive and inefficient. Results of the currently used methods are frequently received after the plant adsorbents have been irreversibly damaged, resulting in an expensive reload of the adsorbent beds. A method is needed that can provide diagnostic results quickly, identify the cause of plant degradation at an early stage and allow plant engineers to effectively perform preventative maintenance procedures to ensure the integrity of the adsorbent materials.